[clang] Make PCH's respect any VFS specified. (PR #106577)

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir commented:

Sorry for being slow to look at this in more detail:

> When generating a PCH map the original directory to some fake directory. You could imagine D:/Foo being mapped to Z:/Foo for instance.

Can you clarify what this means? Is `Z:/Foo` a virtual-only path or does it actually exist on disk? In your test case both directories exist, which I assume wouldn't normally be the case (or else why not build in the desired location directly?).

If the goal is to put virtual paths in the PCH so that you can map them somewhere else in the consuming compiler's VFS, does the`RedirectingFileSystem` setting `'use-external-names': false` do what you need? The idea behind that setting is that we would use the virtual paths everywhere in the compiler instead of translating them to the external/on-disk path. I don't have a lot of experience with enabling that in modules/pch so maybe there's something that it doesn't handle.

https://github.com/llvm/llvm-project/pull/106577